Atherosclerosis is a chronic inflammatory disease and is a leading cause of mortality in cardiovascular diseases. Recent studies demonstrate that monocytes and macrophage heterogeneity contribute to atherogenic and atheroprotective axes. This proposal focuses on the role that alpha-defensin (-def), a major protein expressed in human polymorphonuclear neutrophils (PMNs), plays in the immune response and genesis of atherosclerosis, a role that has been ignored in existing murine models of atherosclerosis because mice do express -def, and in humans due to the short lifespan of PMNs in the plaque. We have shown that -def is abundant in human plaques indicative of PMNs activation. To study the contribution of -def to chronic inflammation and development of atherogenesis in vivo, we generated novel mice that express human -def in their PMNs bred onto ApoE-/- (D+/+ApoE-/-). In preliminary studies D+/+ApoE-/- mice showed less atherosclerosis and reduced systemic inflammation compared with D-/-ApoE-/- control mice on a high fat diet. This is in line with recent studies that contrast with the widely accepted pro-inflammatory role of -def, documented anti-inflammatory effects on macrophages and monocytes. Moreover, -def inhibits activation of NF-kB in macrophages, inhibits adhesion of human and mouse macrophages to matrix proteins and increases the number of the regulatory resident subset of monocytes in inflammation mouse model. Together, these data support an anti-inflammatory, anti-atherogenic role for -def in vivo. We hypothesized that -def released from activated PMNs in nascent atherosclerotic lesions modulate the monocytes/macrophage subset distribution from a pro-inflammatory to an anti-inflammatory phenotype by inhibiting NF-kB activation and the subsequent expression of multi pro-inflammatory cytokines. To test this hypothesis, we will study the effect of -def on atherosclerosis and inflammation in vivo and elucidate its mechanism of action both in vivo and on monocytes and macrophages using isolated -def and neutrophils packed -def in novel def+/+ mice that we developed. In sum, this proposal is a critical initial step to understand the critical role that -def plays in inflammatio, and will provide a paradigm-shifting from, the widely accepted proinflammatory role of -def, into immune modulator or anti-inflammatory role under conditions of chronic inflammation in atherosclerosis.